Warren Buffett Partying With Jay-Z

81-year-old Warren Buffett — one of the richest men on the planet — got downright gangsta at the grand reopening of Jay Z’s 40/40 nightclub in NYC last night … throwin’ up the Roc-a-Fella sign … like a boss.

It was the first time Jay’s been out in public since the birth of his daughter Blue Ivy — though Beyonce and B.I. didn’t make it out to the party.

Jay and Buffett have been friends for years — and appeared on the cover of Forbes together back in 2010.

Sources tell us … Warren wasn’t scheduled to appear at the event, but called and asked if he could roll by … and Jay was all about it.

We’re told Buffett took a tour of the club and really enjoyed the place.

As for the Roc hand sign — we’re told Buffett noticed people throwing it up in the club, so he asked Jay what it was. Once Jay informed him … the rapper personally instructed him how to make the hand sign … and they threw it up together.